Bitumen is recovered from subterranean oil sand deposits by the utilization of steam at a pressure of about 1500 to 2000 psi to render the bitumen flowable and the removal of the bitumen from the deposit in suspension in hot water under the influence of the steam pressure. After separation of the bitumen from the aqueous phase at the surface, there is left a waste water known as "brackish water". This oil-recovery process is carried out at Peace River, Alberta, Canada by Shell Oil and at Cold Lake, Alberta, Canada by Esso. The brackish water contains considerable mineral concentrations and also residual hydrocarbons both of which inhibit reutilization of the water for steam production, since the minerals scale the reboiler tubes and the hydrocarbons cause fouling of boiler parts. Currently, the waste water stream along with other produced waters are disposed of into deep well subterranean storage.
The volume of water required for steam extraction is substantial, amounting to approximately 3 to 9 barrels of water per barrel of bitumen recovered. The inability to use the brackish water for steam regeneration imposes a considerable strain on the limited resources of the region and also necessitates disposal in an environmentally-safe manner.
In U.S. Pat. No. 4,395,337, assigned to the assignee hereof, the disclosure of which is incorporated herein by reference, there is described a procedure for the treatment of brackish water to remove contaminants therefrom and to render the same useful for reuse in steam generation. As set forth therein, the treatment procedure involves air flotation to remove free oil, treatment with ozone to remove residual hydrocarbons, electrodialysis reversal to remove minerals other than silica, and subsequent removal of silica.
It has now been found that the treatment with ozone leads to unforeseen difficulties. Not all dissolved organics were removed, leading to difficulties in reuse, and some dissolved organics were converted to materials which foul the electrodialysis membrane, leading to a decreased membrane life. Further, it has been found that the air flotation step often was unsuccessful in removing an acceptable proportion of the free oil and other suspended solids from the waste stream, also leading to membrane fouling and a lesser quality of product water.
It has further been found that, in many cases, the brackish water treated was in a reduced state, which inhibited removal of organics and suspended solids, leading to decreased quality of product water.